There has been known a fuel cell system including: a fuel cell stack that generates power by an electrochemical reaction between fuel gas and oxidant gas; a cooling water supply pipe that links an inlet of a cooling water passage formed in the fuel cell stack, and an outlet of the cooling water passage to each other outside the fuel cell stack, whereby a circulation path for cooling water is formed; a radiator that is arranged in the cooling water supply pipe; a radiator bypass pipe that links the cooling water supply pipe between the inlet of the cooling water passage in the fuel cell stack, and the radiator, and the cooling water supply pipe between the outlet of the cooling water passage in the fuel cell stack and the radiator to each other; a cooling water pump that is arranged in the cooling water supply pipe between the inlet of the cooling water passage in the fuel cell stack and the radiator bypass pipe, and delivers cooling water; a three-way valve that controls the amount of cooling water supplied to the radiator, and the amount of cooling water supplied into the radiator bypass pipe, from cooling water flowing from the cooling water passage in the fuel cell stack into the cooling water supply pipe; and an ion exchanger that is arranged in the radiator bypass pipe, and performs ion exchange of cooling water (see, for example, Patent Literature 1).
Use of a thermostat valve as the three-way valve, which is not clearly described in Patent Literature 1, is commonly performed. The opening degree of the thermostat valve depends on the temperature of the thermostat valve, and the temperature of the thermostat valve depends on the temperature of cooling water supplied to the thermostat valve. In other words, the thermostat valve supplies a total amount of the cooling water to the radiator when the temperature of the thermostat valve is high, i.e., when the temperature of the cooling water is high. As a result, the cooling water is cooled in the radiator, and therefore, the temperature of the cooling water is decreased. In contrast, the thermostat valve supplies the total amount of the cooling water to the radiator bypass pipe when the temperature of the thermostat valve is low, i.e., when the temperature of the cooling water is low. As a result, the cooling water is not supplied to the radiator, and therefore, the temperature of the cooling water is inhibited from decreasing.
Because ionic impurities are gradually mixed from the radiator, the pipes, and the like into the cooling water, the conductivity of the cooling water gradually increases. The high conductivity of the cooling water may cause liquid junction and the like, and is unfavorable. Thus, in Patent Literature 1, the ion exchanger that can remove ionic impurities from the cooling water is arranged in the radiator bypass pipe.